if you sheet with balsa, you won't need FG. it's just needless added weight IMHO. if you want FG, I would cut a core, sand to shape, spackle, then FG. the problem with a cut core, is that you've got to hollow out sections for servos, flap recesses, aileron recesses, etc.. if it were me, I would do a built up wing using foam ribs, with balsa, hard wood and CF for stringers and spars. with a wingspan this large, I would be very suspect of the structural integrity of a foam core. if you look at super large scale builds, more often than not, they use a built up wing because A) it's lighter than a foam core, and B) it's stronger than a foam core of the same size. the bonus is that you can build everything right into the wing and not have to reverse engineer a cut foam core to make everything fit right.

this is your decision to make here, but thought I'de point out some pros and cons of foam verses built up. the obvious pro to a cut foam core, is the ease of manufacture. it's the easy way out, but not the best way out. there's lots of ways to do a built up wing, if you think it through and design it on paper, or computer.

how about hotwiring my wing cores then sheeting the with balsa and give the top surface a coat of fibreglass

Keenan thats a great ideah if you Hot-wire the 225's wings & stabilizer & twin rudders your 225 build will be possibile, try and find a UK RCG member that has a Hot-wire you can borrow. I would also ask Pontious how he would build your An-225 he's great at making large Depron jets.

an even better option may be to do a built up center section and a hot wired outer section. the beauty of this solution, is that you can have remaovable outer sections that can be attached via aluminum tube. the wing essentially becomes a three piece and entirely removable. if I was going this route, I'de do the inner 4 engine pylons as a built up section and the outer portion with a single pylon on each section. to power this setup, you'de wire the inner pair of fans, the middle pair of fans and then the outer pair of fans. the outer pair of fans could be set up in such a way, that they draw their power off the same system as one of the other pairs, but they only draw power after the throttle is above say, 75% or 80% position. so, for flight, you run off four fans and for takeoff, etc., you are using all six. just a thought I was having to increase your flight time.

it might be a good option to mount the ESC's behind each fan. since you aren't pushing a lot of power on 4S or 5S, you shouldn't be worried about wire length. on 4S, I ran a wire 22 inches from the battery to the ESC connector. the battery was slightly warmer than when it was the stock length, but not by much. ideally, you want the motor to ESC wires a bit longer, but at these power levels, it's not a big deal which wires you extend. if it were me, I'de mount the ESC's behind each fan and run the wires up into the wings. mount the packs in the wings if there's enough room, or further on into the wing root somewhere. just a thought.

and at $12 per servo they should'nt burn a hole in my pocket (only using 9g servo's on the spoilers
My freinds airframe will have an assortment of his of his own servos

Not a Bad MG servo K, Hobbyking has something similar, They sell them for $10 a piece, just remember they weight of these servos adds up, if your not carefull you could wind up with a pound or more servos !